While Hillary Clinton is still better positioned to win the presidency than Donald Trump, her margin for error has shrunk -- even though she was widely perceived to have won the first debate.

Currently, we project that Clinton has 263 electoral votes and Trump has 216. Clinton's total is down from the 272 and 280 she had in our August and March handicapping, respectively. Trump's total is up from 181 in August and March.

But perhaps more important, the Democratic candidate is now below the crucial 270 electoral vote threshold that she needs to win the presidency. That means she would have to win one or more of the four tossup states to capture the White House.

According to our analysis, some combination of four tossup states -- Colorado, Florida, Nevada and North Carolina -- will now determine which candidate wins.

Three of these four states had already been included in our tossup category in August. The newcomer is Colorado and its nine electoral votes. While one poll released in early October had Clinton ahead in the state by double digits, that followed a month of tight survey results.

Our assessment -- based on interviews with political analysts as well as recent polling data -- categorizes states as either safe Republican, likely Republican, lean Republican, tossup, lean Democratic, likely Democratic or safe Democratic. Within each category, the states run from most likely to vote Republican to most likely to vote Democratic.

The recent shift to Trump stems from his success in several states we previously considered tossups. Each of these states remains a battleground in our analysis, but they now tip modestly toward Trump.

The most striking of these shifts is Ohio, the fabled bellwether state. In the three election cycles since we've been handicapping the Electoral College, Ohio has never left the tossup category -- until now. In recent weeks, Trump has made gains in the polls thanks to favorable demographics -- a sizable white, working-class population that is receptive to his message. As a result, Ohio has shifted to lean Republican.

Iowa is another state we're moving from tossup to lean Republican. It voted twice for Barack Obama, but it has a significant population of white working-class voters and relatively few minorities, a group more favorable to Clinton.

Until recently, we had placed Missouri in the tossup category even though the onetime swing state had slipped into the GOP column in recent presidential cycles. But after some close polling early in the campaign, the state's presidential preference appears to be reverting to its recent redder tinge, so we're moving it to lean Republican.

The final switch that boosted Trump's electoral vote count in our analysis involves Maine, a state in which the statewide vote winner gets two electoral votes and the winner of the vote in each of its two congressional districts gets one electoral vote each.

Previously, we had rated Maine's 2nd congressional district a tossup, but more recent polling shows a Trump advantage in the high single digits. So we're moving it to lean Republican. At the same time we have moved the electoral vote for the state's 1st congressional district -- which is more solid territory for Clinton -- to safe Democratic, while the two statewide electoral votes remain in the lean Democratic category.

During Trump's September surge, the polls narrowed significantly in several states we have been categorizing as lean Democratic, notably Michigan, New Hampshire, Pennsylvania, Virginia and Wisconsin. While we are keeping these states in the lean Democratic category for now, we're relieved that we didn't shift them to likely Democratic in August. At the time, Clinton was registering high single-digit or even double-digit polling leads.

Of them, Pennsylvania, with 20 electoral votes, is especially pivotal. A Trump victory there -- potentially enabled by historically strong margins among blue-collar white voters in the southwestern part of the state -- could vastly ease his path to the White House.

But based on discussions with experts in the state, we're keeping Pennsylvania in the lean Democratic category, albeit in the spot adjoining the tossup category. Trump faces a challenge wooing voters - even Republicans -- in the populous and more affluent southeastern counties surrounding Philadelphia.

Similarly, polling margins have narrowed in Michigan, New Hampshire, Virginia and Wisconsin. But polling taken after the first debate shows that Clinton has recovered some of the ground she had lost earlier in September, so we're keeping these states in the lean Democratic column for now.

While Clinton has kept one red state from 2012 in serious contention -- North Carolina -- she has not moved the needle much in two other historically red states that seem like potential targets for her in 2016: Arizona and Georgia. So we're keeping them in the lean Republican category for now.

While Clinton's edge over Trump has shrunk in our current assessment, it's important to note that by recent historical standards, her edge remains reasonably healthy.

In our final handicapping of the 2012 cycle, Obama had 237 electoral votes leaning his way -- significantly fewer than Clinton has now. And Clinton's current 263 electoral vote total matches what Obama had in our final 2008 handicapping when he defeated U.S. Sen. John McCain of Arizona.

The one electoral vote in Maine's 1st congressional district that shifted toward Clinton is the only one that went in her direction in our new analysis. Every other shift between categories benefited Trump, although many of the switches in Trump's direction are relatively unimportant because they came in states that are not considered key battlegrounds for either party.

For instance, we have moved two solidly red states -- Kansas and Utah -- from likely Republican to safe Republican. While Trump is expected to underperform compared to previous Republican nominees in each of these states, we've concluded that Trump has a big enough cushion in both that he won't need to worry about losing either state.

Meanwhile, on the Democratic side, we're moving three states -- Connecticut, New Jersey and Rhode Island -- from safe Democratic to likely Democratic. We're not doing this because we expect Trump to be genuinely competitive in these historically Democratic states -- he almost certainly won't be. Rather, we're being prudent in the face of some polls that suggest tighter-than-normal margins for a Democratic nominee.

The remaining shifts in our current handicapping involve states that are on the fringes of being competitive but are significantly less crucial than the main battlegrounds.

One is Nebraska, the only state other than Maine that allocates electoral votes by congressional district. We previously classified one of Nebraska's electoral votes as lean Republican, but with Trump's recent gains, we've switched that to likely Republican.

The other two states we've shifted -- Minnesota and New Mexico -- are ones where we still expect the Democrats to win, but where Clinton's current margins are relatively narrow. In New Mexico, for instance, the presence of Libertarian Gary Johnson on the ballot poses a particular threat to Clinton; Johnson previously served two terms as governor of New Mexico and took 24 percent of the vote in one recent poll.

Here is our current state-by-state breakdown. States listed in bold have switched since our August handicapping: